The invention relates to a thermal sensing circuit.
Referring to FIG. 1, a thermal sensing circuit 10 might be used to, for example, monitor a substrate temperature of a microprocessor 12. When the temperature exceeds a predetermined temperature threshold (100.degree. C., for example), the thermal sensing circuit 10 might alert circuitry of a computer system so that corrective action (throttling back or shutting down the microprocessor 12, for example) may be taken to reduce the temperature. Without the corrective action, the microprocessor 12 may overheat and catastrophic failure of the microprocessor 12 may occur.
The thermal sensing circuit 10 typically is fabricated on a separate discrete integrated circuit, or chip, and is coupled to one or more external pins of the microprocessor 12. Using these pins, the thermal sensing circuit 10 typically biases a thermal sensing element (such as a diode 14, for example) of the microprocessor 12 into forward conduction and senses an analog voltage across the thermal sensing element. This analog voltage indicates the substrate temperature, and the thermal sensing circuit 10 may convert the analog voltage into a digital value that is stored in a register of the circuit 10. As described below, the thermal sensing circuit 10 may use this digital value to determine when the temperature surpasses a maximum temperature threshold.
The temperature threshold may be programmed via a threshold register 23 (of the thermal sensing circuit 10) that may be accessed through a system management bus (SMB) 30. As is typical, the SMB 30 includes an SMBCLK clock line and an SMBDATA data line that may used to store data in and retrieve data from the thermal sensing circuit 10. When the temperature threshold is exceeded, the thermal sensing circuit 10 may assert an SMBALERT# signal (which is carried by another line of the SMB 30) to alert the computer system that corrective action is needed.
To accomplish the above-described functions, the thermal sensing circuit 10 typically includes an analog-to-digital (A/D) converter 16 which receives the analog signal (from the sensing element) and converts the analog signal into the digital value. A digital comparator 24 (of the thermal sensing circuit 10) compares the digital value to a value stored in the threshold register 23 and stores the result of the comparison in a status register 26.
At least two factors may affect the accuracy of the thermal sensing circuit 10, and the A/D converter 16 governs both of these factors. First, the rate at which the thermal sensing circuit 10 samples the temperature of the substrate may not be fast enough to track the microprocessor's temperature in real time. Although a typical measurement rate may be two samples per second (i.e., 2 Hz), the thermal sensing circuit 10 may need to be capable of measuring the temperature at a minimum rate of eight samples per second in order to handle a wide range of thermal management solutions. The sampling rate depends on how fast the temperature may change (i.e., the sampling rate must be greater than or equal to the Nyquist rate), and the required sampling rate may increase as the microprocessor 12 dissipates more power.
Another factor affecting the thermal sensing circuit's accuracy may be the accuracy of the A/D converter 16. In this manner, in order for the thermal sensing circuit 10 to accurately sense the temperature, the A/D converter should exhibit a low quantization error so that the A/D converter 16 does not introduce an error that exceeds .+-.1.degree. C. Since a temperature coefficient of the thermal sensing element may be 2.2 mV/.degree. C., the resolution of the A/D converter 16 should be at least 1 mV to avoid inaccurate readings due to quantization errors.
Thus, there exists a continuing need for a thermal sensing circuit to accurately sense a substrate temperature.